‘I really can’t understand why people want to be famous. It’s the most f***ing revolting thing in the world.’ Irvine Welsh is on a bit of a rant. Globalisation; Cameron and Clegg; super-rich Premier League football teams: according to the author, there’s a lot that’s wrong with the world. ‘Although I can understand the wish to be stinking rich,’ he concedes. ‘I love having money; anyone from my background says the same. That’s what’s driven me.’

Be it money, or what he calls a ‘Calvinist work ethic’, Welsh has come far since his 1993 debut, Trainspotting, blew open the sequestered netherworld of bedsit junkies in living in Leith, Edinburgh. He has written ten books since – though none has been as good – and long ago swapped Leith for Chicago and Miami, where he lives with his wife, Beth.

This week, the film adaptation of Ecstasy hits cinemas, while Filth, starring James McAvoy (‘better than Robert de Niro in Taxi Driver,’ says Welsh), is in post-production. Also on the cards is The Magnificent Eleven, a feel-good flick about an East End football team, which he’s script-developed. ‘However f***ed up I was, I always thought of heroin as an interesting diversion,’ says Welsh. ‘I never regarded it as my final destination.’

Still, those early days of shooting up in Leith are precisely where Welsh, aged 53 and clean for years, is now returning. This week, he publishes Skagboys, a 548-page prequel to Trainspotting, part of which he retrieved after stumbling across Trainspotting’s original draft on old Amstrad discs. Renton, who opens the book recounting a trip with his dad to support the miners at Orgreave, South Yorkshire, in 1984, is at Aberdeen university; at home, Thatcher’s cuts are biting, unemployment is rocketing and skag has hit the streets.

‘I wanted to show how Sick Boy and Renton got to that point,’ says Welsh, who added dispatches on unemployment, Aids and the rumours surrounding alleged security lapses at a pharmaceutical factory in Edinburgh believed by some, he says, to have helped Edinburgh’s smack invasion in the 1980s. ‘They weren’t always junkies. They had a family, ambitions.’

Skagboys quickly resembles Trainspotting, as Renton and Sick Boy descend into the haunted twilight of smack addiction, but without its incendiary zeitgeist power. However, few write better about drug dependency and some of his descriptions of delirium, ecstasy and need are as good as anything he’s written. But Welsh doesn’t think he could write today’s version of Trainspotting. Not because things have changed – but because they haven’t.

‘The atmosphere is probably as bad as it’s ever been for the past 30 years,’ he says. ‘The kids in the 1980s came from full employment. They had a status as a working class, who were going to work hard. Now, you’ve got three generations who’ve had nothing. All that hope has gone. You wouldn’t have that same cockiness. You’d just have people f***ing each other over.’

This is pretty bleak, even from an author who has devoted his career to exploring the depths of base human behaviour. Yet he’s never regarded himself as a nihilistic writer. ‘Renton flirts with nihilism,’ he says, ‘but I’m more interested in failure. How people wilfully make the decision to f*** up.’

Sometimes, writing about people f***ing up f***s him up too. ‘All those embarrassments and humiliations come back,’ he says. ‘Filth [featuring Welsh’s most monstrous character, DS Bruce Robinson] and Crime [about a paedophile ring] were very difficult because you get into the character to some extent. With Bruce, I wasn’t right for about a year. I have certain rituals, though. I make myself do something normal for a bit and I’m back to being Irvine.’

Skagboys (Jonathan Cape) is published on Thursday, priced £12.99. Irvine Welsh appears at The Scala, London, tonight and The Caves, Edinburgh, on Friday. http://www.bookslam.com